denyhosts/clamav/libclamav/tomsfastmath/bit/fp_div_2d.c

/* TomsFastMath, a fast ISO C bignum library.
 * 
 * This project is meant to fill in where LibTomMath
 * falls short.  That is speed ;-)
 *
 * This project is public domain and free for all purposes.
 * 
 * Tom St Denis, tomstdenis@gmail.com
 */
#include <tfm_private.h>

/* c = a / 2**b */
void fp_div_2d(fp_int *a, int b, fp_int *c, fp_int *d)
{
  fp_digit D, r, rr;
  int      x;
  fp_int   t;

  /* if the shift count is <= 0 then we do no work */
  if (b <= 0) {
    fp_copy (a, c);
    if (d != NULL) {
      fp_zero (d);
    }
    return;
  }

  fp_init(&t);

  /* get the remainder */
  if (d != NULL) {
    fp_mod_2d (a, b, &t);
  }

  /* copy */
  fp_copy(a, c);

  /* shift by as many digits in the bit count */
  if (b >= (int)DIGIT_BIT) {
    fp_rshd (c, b / DIGIT_BIT);
  }

  /* shift any bit count < DIGIT_BIT */
  D = (fp_digit) (b % DIGIT_BIT);
  if (D != 0) {
    register fp_digit *tmpc, mask, shift;

    /* mask */
    mask = (((fp_digit)1) << D) - 1;

    /* shift for lsb */
    shift = DIGIT_BIT - D;

    /* alias */
    tmpc = c->dp + (c->used - 1);

    /* carry */
    r = 0;
    for (x = c->used - 1; x >= 0; x--) {
      /* get the lower  bits of this word in a temp */
      rr = *tmpc & mask;

      /* shift the current word and mix in the carry bits from the previous word */
      *tmpc = (*tmpc >> D) | (r << shift);
      --tmpc;

      /* set the carry to the carry bits of the current word found above */
      r = rr;
    }
  }
  fp_clamp (c);
  if (d != NULL) {
    fp_copy (&t, d);
  }
}

/* $Source$ */
/* $Revision$ */
/* $Date$ */